This invention relates to (A) multifilament poly(p-phenylene-2,6-benzobisoxazole) (xe2x80x9cPBOxe2x80x9d) yarn or cord which is coated with a mixture of an epoxy resin with a vinyl pyridine-styrene-butadiene rubber latex (VPSBRL), the mixture referred to as a xe2x80x9csubcoatxe2x80x9d; the subcoated cord is then again coated by dipping in a conventional reaction product of a phenolic compound, an aldehyde donor and a latex, familiarly referred to generically as a xe2x80x9cresorcinol-formaldehyde latex (RFL)xe2x80x9d; and, (B) to a PBO-finishing process to make twice-coated yarn, in which process the epoxy-latex mixture is applied to PBO yarn which may have been given a spin-finish, or corona, or plasma treatment, yielding subcoated PBO yarn; and, the subcoated yarn is then again coated by dipping in a conventional RFL dip. The twice-coated yarn or cord has improved adhesion comparable to that provided by a coated poly(phenylenediamine terephthalamide) xe2x80x9caramidxe2x80x9d yarn. Aramid is the generic name for fiber in which the fiber-forming substance is a long-chain synthetic aromatic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic linkages.
The great strength of yarn or cord made from aramid fibers which have a crystalline surface has resulted in its widespread use to reinforce a variety of rubber articles in which adhesion is of paramount importance. The inherently poor bonding of vulcanized rubber to the surfaces of aramid fibers was overcome, over many years, by using several different processes many of which rely on a polyepoxide, or xe2x80x9cepoxyxe2x80x9d for brevity, subcoat followed by a RFL dip.
PBO yarn has higher strength and could deliver better performance than aramid yarn in reinforced tires, conveyor belts, drive belts and the like, if the inherently poor adhesive characteristics of the PBO yarn, due to low chemical reactivity and rigid surface structure, could be overcome. The outstanding flame resistance and thermal stability of the yarn is ideally suited for the manufacture of heat-resistant fabrics used to make high-pressure high-temperature resistant hose and protective clothing.
Because PBO fiber has a tensile modulus (T/mm2) nearly twice as high as that of aramid fiber it was believed that PBO yarn was ideally suited for use in reinforced sulfur-vulcanizable rubber if the yarn could be successfully coated with a coating which exhibited a comparable or better adhesion for rubber than which has been already achieved in aramid yarn.
The Problem
The surface characteristics of PBO yarn and the filaments from which it is made, are such that it is difficult to obtain substantially the same degree of adhesion with rubber as is currently obtained with aramid yarn. Of the many epoxy resins with which aramid, and some polyester yarns, may be effectively subcoated before each is topcoated with a conventional RFL dip, the aromatic polyglycidyl ethers are not sufficiently effective with PBO yarn. A subcoat for PBO yarn is to be found which has substantially the same adhesion to sulfur-vulcanized rubber as does successfully subcoated aramid yarn.
It is well known that an epoxy resin, both aliphatic and aromatic polyglycidyl ethers, is a highly effective subcoat for aramid fibers and it is unnecessary to combine the epoxy resin with a rubber latex of any kind. Moreover, as will be seen in Tables IV and V below for reinforcement by embedding and bonding twice-coated cord in the same two rubber compounds used in belts or plies, it was found that addition of a nitrile-butadiene rubber (xe2x80x9cNBRxe2x80x9d) latex to either an aliphatic or an aromatic polyglycidyl ether used in a subcoat for PBO cord, failed to provide adhesion comparable to that provided without the NBR latex in aramid cord.
Since it is also known that epoxy resins could be reinforced with PBO fibers as disclosed in U.S. Pat. No. 5,874,152 to Middelman, an obvious choice was to use an epoxy resin as a subcoat. However, as will be evident from tests presented hereafter, several epoxy resins provided reasonably good adhesion in sulfur-vulcanizable rubber when used as subcoats, followed by a RFL dip, but the adhesion to PBO yarn was far from a close match compared to the adhesion provided by aramid cord in sulfur-vulcanizable rubber. Not unexpectedly, it was found that aliphatic polyglycidyl ethers which are effective only in combination with a VPSBRL on PBO yarn, were also effective without the latex, on aramid fibers.
It is known, as disclosed in U.S. Pat. No. 6,077,606 to Gillick et al, that carbon yarn may be used to reinforce a rubber composition comprising VPSBRL in combination with resorcinol, formaldehyde and an acrylonitrile-butadiene copolymer, if the yarn is first impregnated with an aliphatic epoxy resin, but there was no reason to believe that the combination of the VPSBRL with the epoxy resin would provide an effective subcoat for the carbon yarn, or for any other yarn.
Rubber articles designed to withstand high stresses in use are typically reinforced with substantially inextensible yarn or cord derived from filamentary polyester, nylon, glass, graphite, ultra high molecular weight (UHMW) polyethylene, polypropylene, polyvinyl alcohol, aramid and the like, the last named being the current material of choice for high-performance rubber hose, belts, and tires, inter alia. In such articles, it is essential that the yarn or cord be firmly adhered, preferably cohesively bonded, to the rubber and remain effectively adhered even after the article has been repeatedly subjected to strains varying by orders of magnitude in use, because any separation and relative movement of the rubber and yarn or cords leads to abrasion therebetween and failure. When cord or yarn is cohesively bonded to rubber, pulling the yarn or cord out of the rubber results in the rubber being torn away so that it covers a major portion of the surface of the yarn or cord. Twice-coated PBO yarn is particularly desirable for reinforcing conveyor belts, drive belts and any of the rubbery portions of a tire, especially the tread and breaker plies.
PBO yarn from filaments which may be provided with an initial spin-finish, or a corona, or a plasma treatment, is first coated with a subcoat of a mixture of a slightly water-soluble epoxy resin with a vinyl pyridine-styrene-butadiene rubber latex (VPSBRL), then coated in a conventional RFL dip, to yield a twice-coated PBO yarn; this yarn provides adhesive strength in sulfur-vulcanizable rubber which is substantially the same or better than that provided by aramid yarn having the same physical dimensions and construction, in the same application. By xe2x80x9cslightly water-solublexe2x80x9d is meant that the solubility of the epoxy resin in water at 23xc2x0 C. is in the range from about 1% to 15% by weight. Acceptable bonding is indicated by cohesive failure, evidenced on a scale of (0) to (5) by rubber coverage of (5), and a peel force of at least 100 Newtons.
PBO yarn or cord adapted for the reinforcement of rubber articles has a surface coated with a mixture of VPSBRL such as 2-vinyl pyridine-SBR and an aliphatic polyglycidyl ether having a flash point greater than 150xc2x0 C. with only enough OH-groups to be slightly water-soluble.
Though VPSBRL, used by itself as a subcoat, has substantially the same effect as water, irrespective of the solids content of the subcoat and how much solids is deposited on the yarn, it is found that using a subcoat in which the aliphatic polyglycidyl ether (solids) relative to the VPSBRL (solids) is present in a range from 1:3 to 3:1, the VPSBRL solids preferably being present in a minor proportion by weight; when the subcoated yarn or cord is adequately topcoated with RFL, the adhesion produced is comparable to that provided by a commercially used subcoat on aramid yarn similarly topcoated; preferably the VPSBRL solids are present in the range from about 30 to 95 parts by weight per 100 parts of aliphatic polyglycidyl ether solids deposited on the PBO yarn; the total solids of the deposited subcoat is in the range from about 10 ppm to 1% by weight, based on the dry weight of the subcoated yarn.
Yarn made from a polymer having a (p-phenylene-2,6-benzobisoxazole) repeating unit shown below is derived from a condensation polymerization between 4,6-diaminoresorcinol and terephthalic acid. The structure of the repeating unit is: 
The polymer is a rigid-rod, liquid crystal polymer which has a negative thermal coefficient, that is, it expands when cooled. Additional details about the polymer and how it is made are found in the following references which are incorporated by reference thereto: Evers, Thermoxadatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986); 11 Ency. Poly. Sci. and Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley and Sons 1988) and W. W. Adams et al., The Materials Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989).
A dope of the polymer is spun into high tensile strength fibers by known dry jet-wet spin techniques in which the dope is drawn through a spinneret into a coagulation bath. Well known fiber spinning and coagulation techniques produce fibers or filaments each of which preferably has an average diameter typically in the range from about 10 xcexcm to 25 xcexcm of no more than about 50 xcexcm and more preferably no more than about 25 xcexcm. The average tensile strength of a filament is at least 1 GPa, typically more than 2.75 GPa, preferably at least 4.10 GPa. Multiple filaments, from 100 to about 100,000, are then woven into twisted or untwisted yarn, which in turn, is woven into cord.
The optimum denier of the yarn varies depending upon the desired use, typically being in the range from about 50 to 1000 the particular range for tire cord being chosen depending upon the particular tire and where it is to be reinforced. The yarn may be given an additional corona or plasma treatment and lubricated with an oil and provided with an antistatic agent.
Properties of two representative PBO cords are presented in the Table below:
Various modifications of subcoatings which included an epoxy resin have been tested on the PBO yarn and only a substantially water-soluble aliphatic epoxy resin having a flash point greater than 150xc2x0 C. in combination with a vinyl-pyridine-SBR latex (VPSBRL) gave adhesive strengths which closely matched those provided by aramid cord in sulfur-vulcanizable rubber. It is believed that other copolymerizable pyridyl monomers which have a substituent with a reactive double bond will yield results comparable to those obtained with a vinyl-substituent, but the latter is commercially readily available and is the substituent of choice.
Particularly since an epoxy, whether aromatic or aliphatic, is not combined with any latex when aramid or polyester yarn is coated for commercial applications, and each has essentially the same effect on aramid cord (see Example 4 below) there was no reason to look to a combination of any particular epoxy with a latex for a subcoat for PBO yarn. Further, since the effect of a NBR latex in combination with a commonly currently used aromatic epoxy did not improve adhesion provided by the epoxy alone (see Example 6 below), there was no reason to look to combining an aliphatic glycidyl ether with any latex, much less a vinyl pyridine-styrene-butadiene latex. The effectiveness of the aliphatic polyepoxide and the VPSBRL was particularly unexpected because the VPSBRL, by itself as a subcoat, regardless of how much solids it contained, provided so little adhesion (not much better than water alone), on either yarn, as to merit it use being eliminated promptly. Yet, depositing a subcoat in which as little as from about 5 ppm to 300 ppm of VPSBR solids are present, from less than 2% of subcoat solids, provided excellent results on PBO yarn.
Rubbery aqueous alkaline vinyl pyridine copolymer latices are well known. See U.S. Pat. Nos. 2,561,215; 2,615,826; 3,437,122 and 4,145,494. They comprise a copolymer of about 50 to 95% by weight of butadiene-1,3, 5 to 40% by weight of a vinyl pyridine, and 0 to 40% by weight of a vinyl aromatic compound like styrene. Examples of suitable vinyl pyridines are 2-vinyl pyridine, 4-vinyl pyridine, 2-methyl-5-vinyl pyridine and 5-ethyl-2-vinyl pyridine. It is usually preferred to use a latex of a terpolymer of about 60 to 80% by weight of a butadiene-1,3, about 7 to 32% by weight of styrene and from about 4 to 22% by weight of 2-vinyl pyridine. Even more preferred is a terpolymer of about 70% by weight of butadiene-1,3, 15% styrene and 15% 2-vinyl pyridine. Part of the vinyl pyridine copolymer may be replaced with a rubbery butadiene-styrene copolymer and/or a rubbery polybutadiene so long as the relative ratios between the butadiene-1,3 vinyl pyridine and styrene remain as set forth above.
The rubbery vinyl pyridine copolymer and the rubbery polybutadiene or rubbery butadiene copolymer are made in water using free radical catalysts, chelating agents, modifiers, emulsifiers, surfactants, stabilizers, short stopping agents and so forth. They may be hot or cold polymerized, and polymerization may or may not be carried to abut 100% conversion. If polymerizations are carried out with appropriate amounts of chain transfer agents or modifiers and conversions are stopped below 100% conversion, low or no gel polymers are possible. Free radical aqueous emulsion polymerization is well known as shown by: (1) Whitby et al, xe2x80x9cSynthetic Rubber,xe2x80x9d John Wiley and Sons, Inc., New York, 1954; (2) Schildknecht, xe2x80x9cVinyl and Related Polymers,xe2x80x9d John Wiley and Sons, Inc., New York, 1952; (3) xe2x80x9cEncyclopedia of Polymer Science and Technology,xe2x80x9d Interscience Publishers a division of John Wiley and Sons, Inc., New York, Vol. 2 (1965), Vol. 3 (1965), Vol. 5 (1966), Vol. 7 (1967) and Vol. 9 (1968) and (4) Bovey et al., xe2x80x9cEmulsion Polymerization,xe2x80x9d Interscience Publishers, Inc., New York, 1955.
Examples of slightly water-soluble polyepoxides are polyethylene glycol diglycidyl ether, glyceryl diglycidyl ether, diglyceryl diglycidyl ether, diglyceryl triglycidyl ether, trimethylolpropane polyglycidyl ether, pentaerythritol polyglycidyl ether and sorbitol polyglycidyl ether. Less water-soluble but still usable polyepoxides include triglycidyl isocyanurate; 1-epoxyethyl-3,4-epoxycyclohexane; vinyl cyclohexene dioxide; ethylene glycol diglycidic ether; 1,2-propanediol diglycidic ether; 1,3-propanedioldiglycidic ether; 1,3-butanedioldiglycidic ether; 1,4-butanediol diglycidic ether; 2,3-butanediol-diglycidic ether; and the glycidyl ethers of glycerol, erythritol, pentaerythritol, and sorbitol which contain two to three glycidic groups per molecule, for example, the diglycidyl ether of diglycerol, the triglycidyl ether of hexanetriol and so forth.
Aramid yarn used in this and following examples is in the form of cord, 1100 dtex/2, 9xc3x979 tpi (or 354xc3x97354 tpm) obtained from DuPont; and PBO cord having identical physical specifications as the aramid cord, which PBO cord has not been given a corona or plasma treatment, is obtained from Toyobo Co., Ltd., Japan.
A typical subcoat is prepared as follows: 1 gm of the epoxy resin is dissolved in 99 gm of water and 1 gm of the VPSBRL is addedxe2x80x94when the VPSBRL has 41% solids, the solids content of 101 gm of solution is 1.41 gm.
In each case, cord is first passed over rolls into a dip tank containing the subcoat to be used, then dried and cured in successive zones (xe2x80x9cfirst passxe2x80x9d) in a Litzler oven having two zones, under conditions stated for each zone. The dried and cured cord is then passed a second time through the same zones under the same conditions (xe2x80x9csecond passxe2x80x9d) so that the pick-up of solids is in the range from about 10 ppm to 1% by dry weight of the cord.
Conditions for Subcoating Cord in Litzler Oven
The subcoat, upon drying at a temperature in the range from about 100xc2x0 C. to 200xc2x0 C. for from 1 to 10 min, results in the deposit of from about 10 ppm to 1% by weight, preferably from 50 ppm to 0.5%, of subcoat solids on the dried PBO yarn. This subcoat may additionally include a lubricant, such as butyl stearate, ethoxylated long chain alcohols, ethoxylated polysiloxanes and mixtures thereof, in amounts ranging from about 0 to 10% by dry weight. To facilitate application to the yarn, the subcoat is applied from an aqueous solution in which the epoxy resin is present in an amount in the range from about 0.1 to 5% by weight, and the VPSBRL is present as a latex in which the solids content ranges from about 10 to 60%.
The subcoat composition may be applied to the yarn using any suitable means which is selected primarily based on the physical form of the PBO, whether fabric or cord. Typically used are a metered applicator, a kiss roll, spray or foam, singly or in combination; whatever means is used, it is controlled to provide the requisite amount of deposit.
A preferred composition of the subcoat is a mixture of equal volumes of (i) an aqueous solution of 1% epoxy solids and (ii) VPSBRL containing from 40% to 60% solids.
Dry subcoated PBO yarn is topcoated with a water soluble thermosetting resin prepared from reactants consisting essentially of (i) a compound selected from the group consisting of phenol, resorcinol, the cresols, the xylenols, p-tert butylphenol and p-phenyl phenol and mixtures thereof; (ii) an aldehyde donor selected from the group consisting of formaldehyde, acetaldehyde, furfural, paraformaldehyde and hexamethylenetetramine and mixtures thereof; and (iii) a latex, in an amount sufficient to leave, upon drying, solids in the range from about 0.1 to 10 parts by weight dry per 100 parts of twice-coated PBO yarn.
The adhesive RFL topcoat is applied by dipping the subcoated PBO yarn prior to its incorporation into rubber, utilizing conventional techniques known to those skilled in the art of bonding yarn or cord to rubber. It will be recognized that the RFL dip may include other additives commonly employed by those skilled in the art such as, for example, triallylisocyanaurate, blocked isocyanates, active epoxy compositions, and the like. Following application of the RFL coating, the PBO yarn is heated to a temperature in the range from about 100xc2x0 C. to 300xc2x0 C. for from 30 sec to 2 min, and a layer of compounded rubber is applied to and cured on the twice-coated PBO yarn. The resultant cured composite is coated with the solid residue from the RFL. Information on the preparation of the water soluble thermosetting phenolic-aldehyde resins will be found in xe2x80x9cEncyclopedia of Chemical Technology,xe2x80x9d Kirk-Othmer, Volume 15, Second Edition, 1968, Interscience Publishers Division of John Wiley and Sons, Inc., New York, pages 176 to 208; xe2x80x9cTechnology of Adhesives,xe2x80x9d Delmonte, Reinhold Publishing Corp., New York, N.Y., 1947, pages 22 to 52; xe2x80x9cFormaldehyde,xe2x80x9d Walker, A.C.S. Monograph Series, Reinhold Publishing Corp., New York, N.Y., Third Edition, 1964, pages 304 to 344; and xe2x80x9cThe Chemistry of Phenolic Resins,xe2x80x9d Martin, John Wiley and Sons, Inc., New York, 1956.
For the purposes of this description, xe2x80x9ccompounded rubberxe2x80x9d refers to the natural or synthetic rubber compositions which have been compounded with appropriate compounding ingredients such as, for example, carbon black, oil, stearic acid, zinc oxide, silica, wax, antidegradants, resin(s), sulfur and accelerator(s).
Rubber in PBO-reinforced articles for use in tire manufacture and for other purposes may be natural (Hevea, cis-1,4-polyisoprene) rubber, or synthetic rubber which is a conjugated diolefin polymer, or mixtures thereof including reclaimed rubbers. Such synthetic rubbers are polymers of butadienes-1,3, e.g. butadiene-1,3, isoprene, 2,3-dimethylbutadiene-1,3, and of mixtures thereof, and copolymers of mixtures of one or more such butadienes-1,3, with one or more other polymerizable compounds which are capable of forming rubber copolymers with butadienes-1,3.
It is readily understood by those having skill in the art that rubber compositions used in a tire would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants, reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
Other components which may be present in the topcoat composition include tints, fluorescent brighteners, emulsifiers, antifoaming agents, antimicrobial compounds, co-catalysts, flexibilizers such as methacrylates and mixtures thereof. The total amount of solids (i.e., all constituents except water) in the topcoat composition typically ranges from about 1 to about 30% by weight, preferably from about 5 to about 20% by weight.
The aramid yarn or cord to which PBO is compared, is made from filaments under the trademarks xe2x80x9cFiber Bxe2x80x9d, xe2x80x9cKevlarxe2x80x9d, xe2x80x9cDP-01xe2x80x9d, and xe2x80x9cNomexxe2x80x9d, inter alia. The fibers are made from the condensation product of isophthalic or terephthalic acid and m- or p-phenylenediamine.
The following specific examples are given for purposes of illustration. In all instances the coated cord is the type used in the reinforcement of pneumatic tires, specifically, each cord is designated 1100 dtex/2, 9xc3x979 tpi (or 354xc3x97354 tpm). The coated cord is wound on a xe2x80x9cBand Builderxe2x80x9d constructed for the purpose. A swatch of cord about 20 cm long and 5 cm wide from the Band Builder is calendered into different rubber compounds, a first one used for belts in automobile tires, referred to as xe2x80x9cpassenger tiresxe2x80x9d, a second used for plies in aircraft tires, and a third used for belts in high speed automobile tires. Only the main ingredients are listed, it being understood that the compounds may contain numerous conventionally added additives such as surfactants, waxes, fatty acid salts to reduce the surface tension of the latex, oil-based or synthetic defoamers and gum or acrylate thickeners to provide desirable processing characteristics.
Main ingredients of a compound used for belts in tires of passenger automobiles are as follows:
In the tables below, the above xe2x80x9cautomobile compoundxe2x80x9d is referred to as PASSCPD.
Main ingredients of a compound used for belts in high-speed automobile tires are as follows:
non-productive composition comprising,
and a productive composition comprising,
In the tables below, the above compound is referred to as HSTCPD.
Main ingredients of a compound used for belts in aircraft tires, referred to as xe2x80x9caircraft compoundxe2x80x9d, are as follows:
In the tables below, the above compound is referred to as AIRCCPD.